The CAS is a fundamental parameter of flight control, because it represents the ability of an aeroplane to maintain flight. The CAS is an indispensable item of information for the operation of the autopilot systems.
Conventionally, conventional speed is measured by subsystems of on-board sensors and probes, of “Pitot or Pitot-static” type (which also provide altitude information) or of LIDAR type, and their associated pressure sensors. In the event of a measurement subsystem fault, altitude information can be obtained independently, for example using a satellite navigation system (GNSS). On the other hand, no satisfactory solution exists at this time for determining the airspeed of the aircraft in such an eventuality. Aircraft constructors indicate the procedures to be followed in this case, to enable manual piloting of the aircraft.
The document EP 2 348 285 discloses the possibility of computing the airspeed of an aircraft on the basis of knowledge of its ground speed—obtained using a satellite navigation system—and the wind speed. In the event of a fault in the anemometric subsystems, this latter item of information can be estimated on the basis of statistical data or be communicated by a ground station. This solution is not very satisfactory because the estimate of the wind speed value risks being marred by significant uncertainty.
The article by A.C. in't Veld et al. “Real-time Wind Profile Estimation using Airborne Sensors”, AIAA Guidance Navigation and Control Conference, 8-11 Aug. 2011, Portland, Oreg., United States (AIAA 2011-6662) discloses a method for estimating a wind speed profile as a function of altitude in a TMA (Terminal Manoeuvring Area) by Kalman filtering of data acquired by sensors aboard aircraft and transmitted according to the ADS-B protocol. The aim of this method is not to compensate for the fault in or the absence of the anemometric subsystems, but to know the local wind speed at different altitudes from that at which the aircraft is proceeding.